By some accounts, human beings have been aging distilled spirits in wooden containers for almost five hundred years. Despite billions of person-hours of experience, the myriad of chemical reactions responsible for the flavor of wood-aged spirits are not fully understood. Spirits derive their distinct characteristics over time while stored in wooden containers in part by the production and presence of esters. Esters are compounds made by chemically bonding acid molecules and alcohol molecules to form new compounds, often with pleasant aromas and tastes. This process is known as “esterification.” In addition to esterification, wood-aged spirits derive additional characteristics through other processes, including extraction of flavor compounds from the wood container (e.g., the ubiquitous oak barrel). These processes are not necessarily separate or distinct, and can interact with and affect each other.
Traditionally, producing wood-aged distilled spirits has included carboxylic acid esterification, phenolic acid esterification, and the formation of complex esters, including both phenolic and carboxylic acids. Carboxylic acid esters are responsible for the fruity aromas and tastes in distilled spirits. Carboxylic precursor acids are derived from the yeast and bacteria during fermentation. If organic materials are used for the container in which these reactions occur, those materials also influence the process. For example, where a charred or toasted oak barrel is used, carboxylic precursor acids are also derived from thermal decomposition of the oak polymer, hemi-cellulose, found in the inner lining of the barrel. Carboxylic precursor acids are largely responsible for “off-flavors” in distilled spirits. Additionally, a common flaw with a solvent-like “off flavor” is produced by build-up of ethyl acetate in the maturing spirit. Off-flavors are various flavorful or aromatic compounds present in spirits that are often described by connoisseurs using colorful terms (e.g., “sulfury”, “solventy”, “meaty”, “acidic”, “metallic”, “vegetal”, etc.). Phenolic aldehydes also play a major role producing aromas similar to vanilla, pipe tobacco, and smoke. Phenolic aldehydes are largely derived from thermally broken down oak polymers found within the inner lining of the barrel. Complex esters are responsible for complex honeyed aromas in distilled spirits. The complex esters are generally produced from the chemical reactions of both carboxylic and phenolic acids/aldehydes with alcohols during the time in the barrel or other container.
Fischer esterification of fatty acids and alcohol is a well-understood and commonly practiced chemical reaction. A typical laboratory process involves heating a solution of fatty acids and alcohols under reflux in the presence of an acid catalyst. In laboratory settings, strong acids (e.g., sulfuric acid) are typically employed as the catalyst, but this can be incompatible with spirit making where other functional groups are sensitive to stronger acids and where chemical additives are typically prohibited. It has long been known that Fischer esterification can also be completed using weak acid catalysts, but at the expense of relatively slow reaction rates. Where charred or toasted oak barrels are used during the maturation of distilled spirits, weak acids may be gradually extracted from organic material in the walls of the barrel. It typically takes years for esters to accumulate using weak acid catalysts derived from the barrel, although it has been observed that in warmer environments (i.e., modestly heated within the range of normal atmospheric conditions less than 120° F.) the process can be accelerated significantly (from decades to years). This early form of accelerated aging comes at the cost of increased evaporation from the barrel. Often as much as 50% of the product can be lost to evaporation.
Wood extraction is the process that gives distilled spirits their color and astringent “oaky” and “smoky” taste. Traditionally, this is attributed to tannins (polyphenols). Interestingly our analysis of mature spirits did not find significant evidence of tannins. But instead found myriad less complex wood derived phenols such as sinapaldehyde and syringaldehyde. These oak-extracted compounds proved unexpectedly useful as markers for monitoring the aging process described below.
Attempts have been made to accelerate maturation of distilled spirits by cycling or varying pressures over relatively large ranges (e.g., between −2 and 10 ATM; see U.S. Patent Publication No. 2013/0149423). These processes generally do not yield a product close enough to that produced by traditional means. Other environmental conditions are more important to achieve characteristics associated with a mature flavor.
Esterification and the extraction of wood compounds from the wood container are some of the primary reactions taking place in the maturation process of distilled spirits. Because these processes run concurrently, and often interact with or depend on each other as well as the material and other conditions of their environment over time, it is very difficult to deviate very far from traditional methods while still achieving similar results. For example, rapid oak extraction may not provide enough time for interaction with a wood container to pick up some of the more subtle and complex flavors present in traditionally aged spirits. Much of the expense in spirit making stems from the long latency in creating the end product as well as the product evaporation from the barrel. Stock must be stored, often in climate-controlled environments, and tested repeatedly during maturation. But, it is difficult to predict markets many years out. Makers that produce too much product fail to maximize their investment, whereas those that produce too little fail to capture potentially significant portions of the upside.
Consumers of distilled spirits are often educated and discerning. Many will refuse to consume or pay a premium for non-authentic tasting products. What is needed is a means by which the quality and complexities associated with traditionally aged spirits can be achieved in a significantly reduced timeframe, preferably with a reduction in the evaporation of finished goods and a reduction of the build-up of ethyl acetate.